1. Field of the Invention
The present invention relates to the field of vapor-phase deposition. This invention further relates to a gas delivery system for epitaxial growth on a substrate. The invention also relates to a reaction assembly for generation of a compound gas stream useful in hydride vapor-phase epitaxy. This invention still further relates to methods of delivering a reagent gas to a substrate for vapor-phase deposition.
2. Background of the Related Art
Hydride vapor-phase epitaxy (HVPE) remains an important technique for the epitaxial growth of various semiconductors, such as gallium nitride (GaN). In such a system, growth proceeds due to the high-temperature, vapor-phase reaction between the gallium chloride (GaCl) and ammonia (NH.sub.3). The ammonia is supplied from a standard gas source, while the GaCl is produced by passing hydrogen chloride (HCl) gas over a heated liquid gallium (Ga) supply heated from 750 to 850.degree. C. The GaCl thus produced must be delivered to a substrate, typically through an assembly of tubes.
One major difficulty associated with prior art GaCl generation and transportation, e.g., in a HVPE system, is the unwanted deposition of solid GaCl on various components of the system. GaCl has a high vaporization temperature of more than 500.degree. C., and will thus tend to deposit on any surface that is below this temperature. Such random deposition reduces the quantity of useful GaCl that is delivered to the substrate for reaction with ammonia, and thus reduces the amount of GaN available for epitaxial growth. In addition, deposits of GaCl tend to build up in the gas delivery system, eventually blocking efficient flow of reagents. Unwanted deposition is a particular problem in HVPE systems, where the epitaxial growth rates on the substrate are large, since large amounts of reagents must be transported within the system. As a result, frequent cleaning of the system is necessary to remove unwanted deposits. Cleaning and maintenance of HVPE systems and equipment components is a difficult, costly, time-consuming, and hazardous task. Moreover, where it is desired to deposit a thick layer of GaN on a substrate, unwanted deposition can reach problematic levels during the course of a single growth cycle, whereas cleaning and maintenance of the HVPE system can only be performed after a growth cycle is complete.
Prior art methods for avoiding deposition of GaCl in a HVPE system have involved heating all system components, e.g., tubes, lines, nozzles, to high temperatures. This complicates the design of the system, leading to operational problems and increased costs. In particular, the use of high temperatures to prevent GaCl deposition restricts the design and operation of the system, in that typically the GaCl must be produced in a chamber located very close to the substrate on which deposition is to take place.
Another drawback associated with prior art HVPE systems and methods is that the source or reagent gases often react prematurely, i.e., before reaching the substrate. For example, GaCl and ammonia tend to combine to form GaN on surfaces other than the substrate. Such premature deposition not only decreases the growth rate of the epitaxial layer, but also leads to unwanted deposition of GaN, e.g., on the walls of the reactor or growth chamber. Deposition of unwanted GaN can lead to clogging of the system, and also limits the distance between the GaCl production chamber and the substrate. Prior art attempts at avoiding premature, or non-target, deposition have focused on maintaining the Ga well below the reaction temperature of 1000-1100.degree. C. However, this is difficult, especially since it must be done together with heating above the GaCl evaporation temperature.
The present invention overcomes problems associated with premature or non-target deposition of both reagent (e.g., GaCl) and reaction product (e.g., GaN) in prior art vapor-phase chemical deposition systems and methods.